1. Field of the Invention
The present invention relates to aqueous polyisocyanate crosslinking agents which are modified with monohydroxy carboxylic acids and blocked with pyrazole-based compounds, and to their use as a clear coating composition or a primer surfacer for coating automobiles.
2. Description of the Prior Art
Pyrazoles, e.g., 3,5-dimethylpyrazole, are one of the few isocyanate blocking agents which are stable in aqueous media and capable of forming reactive crosslinking agents. For example, an aliphatic polyisocyanate which is blocked with 3,5-dimethylpyrazole crosslinks under mild stoving conditions of 30 minutes at 120 to 130.degree. C.; whereas, a polyisocyanate which is blocked with butanone oxime requires 30 minutes at 140.degree. C.
Aqueous polyisocyanate crosslinking agents, which are blocked with pyrazoles, are described, e.g., in WO 97/12924, wherein the compounds that impart hydrophilic properties to the crosslinking agent are restricted to polyhydroxy carboxylic acids. The use of monohydroxy carboxylic acids to impart hydrophilicity is not disclosed.
It is known from EP-A 576,952 and EP-A 566,953 that polyisocyanates which are modified with hydroxypivalic acid and polyisocyanates which are blocked with .epsilon.-caprolactam, diethyl malonate, acetoacetic ester or oximes can be used for the production of water-dispersible polyisocyanate mixtures.
EP-A 802,210 also describes aqueous polyisocyanate crosslinking agents which are blocked with 3,5-dimethylpyrazole. Hydrophilic properties are imparted to these crosslinking agents with non-ionic polyethylene oxide chains. However, it has been shown that coating compositions containing these crosslinking agents possess permanent hydrophilic properties and, therefore, cannot withstand a salt-spray test. The corrosion resistance of these coatings is unsatisfactory for the automobile industry.
An object of the present invention is to eliminate the aforementioned disadvantages and to provide an aqueous polyisocyanate crosslinking agent which exhibits good reactivity, which can be synthesized in a simple manner and which exhibits good corrosion-resistance.
This object may be achieved with the polyisocyanate crosslinking agents according to the invention, which are described hereinafter.